1. Field of the Invention
The present invention relates to a magnetic-field producing apparatus for producing a uniform, parallel magnetic field. More particularly, this invention relates to a type of magnetic field producing device which comprises a type II superconductor and a magnetic-field source such as an electromagnet, a permanent magnet or the like. This device can be used in applications such as a nuclear magnetic resonance-computed tomograph (MRI) and the like which require uniform and parallel magnetic fields.
2. Description of the Related Art
In a device such as an MRI or the like which requires a magnetic field of a high degree of uniformity, electromagnets and permanent magnets are used for the production of this magnetic field. The electromagnets of this kind include superconducting electromagnets in which superconducting wires are used and normal-conducting electromagnets in which copper wires or aluminum wires are coil-wound.
For a magnetic field which is below 0.3 T (tesla), the device which uses the permanent magnets will generally be an economical system. However, if a magnetic field over 0.3 T is required, only a superconducting electromagnet can produce such a magnetic field. In addition, since a superconducting electromagnet can produce a magnetic field having excellent uniformity and stability, it is possible to obtain a uniform, high-intensity, time-stable magnetic field (up to several Ts). Therefore a superconducting electromagnet is generally used as a magnet for an MRI.
In the above-mentioned super- or normal-conducting electromagnet, a solenoid form, a double Helmholtz form or the like is adopted for the coil structure and each of these forms can be varied as required.
A sectional view of an example of a known device which produces a uniform, parallel magnetic field by using superconducting electromagnets is shown in FIG. 1. As shown in FIG. 1, superconducting coils 2 are placed at both axial ends of a ring-shaped, very low 10 temperature vessel (referred to as a "cryostat" hereinafter) 1. The magnetic fields emitted by the superconducting coils 2 include curved components extending from the vicinity of the central portion of a cylinder axis 4 toward the outside of this cylinder axis. In order to make those magnetic fields parallel to the cylinder axis 4, another superconducting coil 3 is placed in the vicinity of the central portion of the cylinder axis 4. In order that a given central region of the magnetic fields produced by the superconducting coils 2, 3 is made generally parallel to the cylinder axis 4, the positions and shapes of these coils are suitably determined, and the currents supplied to these coils are appropriately adjusted. Thus, the distribution of the magnetic flux (F) generated by the superconducting coils 2, 3 has the form shown in FIG. 2.
On the other hand, the production of uniform and parallel magnetic fields by making use of perfect diamagnetism of superconductors has been proposed for a long time. For example, Williams et al. demonstrated the effectiveness of this concept through their experiments (W. L. Williams et al.: Phys. Lett. 9 (1964) 102). Furthermore, in the above-mentioned literature, Onsager reported in private that it is possible to produce a uniform magnetic field by using a sheet of a superconductor wound in a spiral shape. Moreover, Hechtfisher demonstrated that it is possible to produce a uniform magnetic field by using a superconductor foil rolled up one layer over another in the form of a hollow cylinder (D. Hechtfisher: J. Phys. E: Sci. Instrum. 20 (1987) p.143).
According to the above-mentioned method proposed by Williams et al., it is possible to produce a uniform, time-stable magnetic field. On the other hand, according to the above-mentioned method proposed and demonstrated by Hechtfisher, it is possible to produce a uniform, intensity-variable magnetic field.
These methods produce uniform magnetic fields or make magnetic fields uniform according to the following principles.
When a superconductor is perfectly diamagnetic, that is to say, when the magnetic field is below its critical magnetic field (Hc) if the superconductor is a type I superconductor, or when the magnetic field is below its lower critical magnetic field (Hc.sub.1) if the superconductor is a type II superconductor, this superconductor exhibits a Meissner effect, and the magnetic flux density (B) within the superconductor becomes zero. This can be expressed in the form that the magnetic permeability (.mu.) of the superconductor is equal to zero.
A type I superconductor assumes its normal conducting state when a magnetic field applied to it is greater than its critical magnetic field (Hc). The materials of the type I superconductor include In (Hc(0K)=0.03 T, Sn (Hc(0K)=0.03 T), Pb (Hc(4.2K)=0.08 T) and the like. On the other hand, a type II superconductor is perfectly diamagnetic when a magnetic field applied to it is less than its lower critical magnetic field (Hc.sub.1). When the applied magnetic field is greater than this lower critical magnetic field (Hc.sub.1), this gives rise to a mixed state in which the magnetic field has penetrated into the superconductor in the form of quantized magnetic flux. As a result, this superconductor is still in its superconducting state but has lost its perfect diamagnetism. Furthermore, when the applied magnetic field reaches the upper critical magnetic field (Hc.sub.2), the superconductor returns to its normal conducting state. The materials which show such characteristics include Nb--Ti (Hc.sub.1 (4.2K)=up to 0.01 T, Hc.sub.2 (4.2K)=up to 10 T), Nb.sub.3 Sn, Y--Ba--Cu--O (Hc.sub.1 (0K)=up to 0.04 T, Hc.sub.2 (0K)=up to 100 T) referred as a "high-temperature oxide superconductor", Bi--Sr--Ca--Cu--O and the like.
The normal component of magnetic flux density (B) which passes the boundary surface between two materials whose magnetic permeabilities (.mu.) are different from each other, must be continuous across the boundary surface. Therefore, when the surface of the superconductor is in contact with a vacuum, only the magnetic field (i.e., magnetic flux density) which is parallel to the superconductor surface can exist in the vacuum, since magnetic flux density (B) inside the superconductor is equal to zero. This is explained by reference to FIG. 3A and FIG. 3B.
In each of these figures, a superconductor is in contact with a vacuum on the boundary indicated by the line segment A-A'.
In the case of FIG. 3A, since the normal component of magnetic flux density (B) which is perpendicular to the boundary surface (i.e., the surface of the superconductor) must be continuous across the boundary surface, this normal component must also exist inside the superconductor as shown by the arrow in the figure, which is contradictory to the requirement that the magnetic flux density (B) must be equal to zero. In other words, only when the magnetic flux density (B) is parallel to the boundary surface as shown in FIG. 3B, does it become possible to satisfy both the requirement that the magnetic flux density (B) must be equal to zero and the requirement that the normal component must be continuous across the boundary surface.
The above-mentioned methods rectify magnetic flux flows of a whole magnetic field system and produce a uniform magnetic field by incorporating this boundary condition into the magnetic field system.
Furthermore, the above-mentioned boundary condition is satisfied only when the superconductor is perfectly diamagnetic.
Therefore, by making use of this boundary condition and by cooling a cylinder of a superconductor material under an axial magnetic field to place this cylinder in its superconducting state, it is possible to trap magnetic flux inside the cylinder or to produce a uniform, parallel magnetic field inside the cylinder.
For example, in a completely hollow cylinder 21 made of a superconductor material as shown in FIG. 4, the magnetic flux indicated by the vector B is trapped in the bore of this cylinder. FIG. 5 is a sectional view which shows the axial distribution of magnetic flux F in this cylinder 21. As shown by this figure, a uniform magnetic field is produced inside the cylinder 21.
Furthermore a cylinder 22 comprising a sheet of superconductor material wound in a tight spiral is shown FIG. 6A. A magnetic field is applied in parallel to the axial direction of the cylinder 22 so that the magnetic field inside the bore of this cylinder can be made uniform.
Furthermore a cylinder 23 comprising a superconductor foil rolled up several times in the form of a hollow cylinder is shown in FIG. 7. FIG. 8 shows the situation in which a magnetic field is applied parallel to the axial direction of the cylinder 23 shown in FIG. 7 so that the magnetic field inside the bore of this cylinder can be made uniform. In this arrangement, a solenoid coil 11 is placed outside the cylinder 23 and the magnetic field is formed parallel to the axial direction of the cylinder 23.
Among these methods, in the method in which a magnetic field is trapped inside the cylinder 21, the magnetic field trapped inside this cylinder has a high degree of time stability, since the magnetic flux which intersects a closed curve inside the superconductor (for example, a circle perpendicular to the cylinder axis) is temporally invariable (this method will be referred to as the "Williams' method" hereinafter). On the other hand, when a cylinder comprises a sheet of a superconductor wound spirally or rolled up several times in the form of a hollow cylinder, it is possible to vary a magnetic field inside the superconductor cylinder 22 or 23, for example, by varying the electric current supplied to the outer magnet, since there is no closed loop which is perpendicular to the axis of the cylinder 22 or 23 (this method will be referred as "Onsager's method" hereinafter).
The above-mentioned methods for producing a uniform magnetic field have several technical, economic and practical problems.
First of all, a superconductor wire used for the composition of a superconducting electromagnet is required to satisfy several strict conditions.
For example, since the superconductor wire is a long wire, it is necessary that the critical current density (Jc) in its longitudinal direction be highly uniform and that the tolerance of the wire diameter be small. Moreover, an extremely sophisticated winding technique is required because the coil portion of the superconducting wire has a very complicated structure. In addition to this, since any slight deformation caused by cooling can be problematic, complicated mechanisms and complex procedures such as numerical calculation, etc., are needed in order to compensate for the degradation of the magnetic field caused by the deformation of the coil portion and to compensate for an undesired magnetic field which cannot be canceled by a solenoid coil.
Furthermore, for an MRI application, an apparatus whose center magnetic field has a magnetic flux density of 1.5 T is currently required. In this type of apparatus, an active shield technique is adopted in order to overcome such hindrances as installation weight and leakage magnetic field, which increases the necessary quantity of the superconductor wire. This forces up the manufacturing cost of the apparatus and hinders the commercial applicability.
For example, such a superconducting magnet is conventionally formed in the manner shown in FIG. 1 for the production of a uniform, parallel magnetic field. However, as shown in FIG. 2, in this arrangement it is difficult to achieve a uniform, parallel magnetic field in a broad area along the axial direction of the cylinder by adjusting the position and shape of the superconducting coil and by adjusting the electric current supplied to this coil. For example, the axial length of the usable area of the magnetic field is less than about one-fourth the axial length of the superconducting coil installed and, on the other hand, the radial length of this usable area is less than about one-half the radius of the superconducting coil. As an inevitable result, an apparatus using such a superconducting coil configuration must be bulky.
On the other hand, the arrangement according to the above-mentioned Williams' method or Onsager's method which uses the perfect diamagnetism of a superconductor is simpler than the arrangement shown in FIG. 1 and makes it possible to expand the usable area of a uniform magnetic field. In other words, since the size of a superconducting coil can be reduced, it is possible to lower the manufacturing cost of such an apparatus. However, the magnetic flux density of a magnetic field produced by means of these methods cannot, by way of example, exceed 50 mT, which makes it impossible to use these methods in order to produce a uniform, strong magnetic field. This difficulty is caused by the adoption of Pb which is a type I superconductor as a superconductor material, because the Hc (which is the upper limit permitting the perfect diamagnetism of the superconductor) of this Pb is relatively high and is about 0.08 T at the liquid helium temperature of 4.2K which is practical for industrial purposes.
For a well known type II superconductor which exhibits superconductivity at a temperature of 4.2K, its Hc.sub.1 (4.2K) which is the upper limit permitting its perfect diamagnetism is lower than the Hc of Pb. Furthermore, in the case of a high temperature oxide of a type II superconductor which exhibits superconductivity even at the liquid nitrogen temperature of 77.3K which is practical for industrial purposes, its Hc.sub.1 at 77.3K is lower than the Hc of Pb at a temperature of 4.2K. This results from the physical property values, such as Hc and Hc.sub.1, and, therefore, is an inherent problem of the material.
In addition, an effective magnetic field which actually influences a superconductor becomes higher than an applied magnetic field because of a diamagnetizing field. For this reason, the superconductor may reach its Hc and lose its perfect diamagnetism even under a relatively low magnetic field. Therefore, in the above-mentioned example of Pb, the above-mentioned methods are considered to be useful only under an applied magnetic field which is lower than the above-mentioned values. (See the above-mentioned article by Williams et al., the above-mentioned article by Hechtfisher or general textbooks written on superconductivity--e.g. Michael Tinkham: Introduction to Superconductivity (McGraw-hill, 1975).)